Week 3 Solutions

Author: hoyeongkwak

combination-sum/hoyeongkwak.py

@@ -0,0 +1,36 @@
+'''
+Time complexity : O(n ^ (target / min_val))
+Space complexity : O(target / min_val)
+backtracking 
+c = [2], total = 2
+c = [2, 2], total = 4
+c = [2, 2, 2] total = 6
+c = [2, 2, 2, 2] total = 8, 초과 backtrack
+c = [2, 2, 2, 3] total = 9, 초과 backtrack
+c = [2, 2, 2, 6] total = 12, 초과 backtrack
+c = [2, 2, 2, 7] total = 13, 초과 backtrack
+c = [2, 2, 3] total = 7 정답 추가
+c = [2, 2, 6] total = 10, 초과 backtrack
+c = [2, 2, 7] total = 11, 초과 backtrack
+....
+[2,2,3], [7]
+
+'''
+
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        result = []
+
+        def backtrack(start, current, total):
+            if total == target:
+                result.append(list(current))
+                return
+            if total > target:
+                return
+            
+            for i in range(start, len(candidates)):
+                current.append(candidates[i])
+                backtrack(i, current, total + candidates[i])
+                current.pop()
+        backtrack(0, [], 0)
+        return result

decode-ways/hoyeongkwak.java

@@ -0,0 +1,36 @@
+/*
+Time complexity : O(n)
+Space complexity : O(n)
+dp를 사용
+현재 숫자만 사용(1~9)
+이전 숫자와 합쳐서 사용(10~26)
+ex) 226
+2 2 6
+22 6
+2 26
+*/
+class Solution {
+    public int numDecodings(String s) {
+        if (s == null || s.length() == 0 || s.charAt(0) == '0') {
+            return 0;
+        }
+
+        int n = s.length();
+        int[] dp = new int[n + 1];
+
+        dp[0] = 1;
+        dp[1] = 1;
+
+        for (int i = 2; i <= n; i++) {
+            int oneDigit = Integer.parseInt(s.substring(i - 1, i));
+            if (oneDigit >= 1 && oneDigit <= 9) {
+                dp[i] += dp[i - 1];
+            }
+            int twoDigits = Integer.parseInt(s.substring(i - 2, i));
+            if (twoDigits >= 10 && twoDigits <= 26) {
+                dp[i] += dp[i - 2];
+            }
+        }
+        return dp[n];
+    }
+}

maximum-subarray/hoyeongkwak.java

@@ -0,0 +1,23 @@
+/*
+Time complexity : O(n)
+Space complexity : O(1)
+*/
+class Solution {
+    public int maxSubArray(int[] nums) {
+        int maxSum = Integer.MIN_VALUE;
+        int currentSum = 0;
+
+        for (int i = 0; i < nums.length; i++) {
+            currentSum += nums[i];
+
+            if (currentSum > maxSum) {
+                maxSum = currentSum;
+            }
+
+            if (currentSum < 0) {
+                currentSum = 0;
+            }
+        }
+        return maxSum;
+    }
+}

number-of-1-bits/hoyeongkwak.java

@@ -0,0 +1,14 @@
+/*
+Time complexity : O(n)
+Space complexity : O(1)
+*/
+class Solution {
+    public int hammingWeight(int n) {
+        int sum = 0;
+        while (n != 0) {
+            n &= (n - 1);
+            sum++;
+        }
+        return sum;
+    }
+}

valid-palindrome/hoyeongkwak.java

@@ -0,0 +1,13 @@
+/*
+Time Complexity : O(n)
+Space Complexity : O(n)
+*/
+class Solution {
+    public boolean isPalindrome(String s) {
+        if (s.length() == 1) return true;
+        String sLower = s.toLowerCase().replaceAll("[^a-zA-Z0-9]", "");
+        StringBuffer strBuilder = new StringBuffer(sLower);
+        String revStr = strBuilder.reverse().toString();
+        return sLower.equals(revStr);
+    }
+}